Chimney construction with heat controlling minimizing means



A R-ii M, 1953 R. K. THULMAN CHIMNEY CONSTRUCTION WITH HEAT CONTROLLING MINIMIZING MEANS Filed Sept. 18, 1947 MN MW m K W E w R Patented Apr. 14, 195 3 UNITED STATES PATENT OFFICE CHIMNEY CONSTRUCTION WITH HEAT CONTROLLING MINIMIZING MEANS This invention relates to the art of chimney construction and aims generally to improve the same. It is particularly concerned with minimizing the transfer of heat through the chimney walls when the heating device to which it is attached is operating normally so that a draft may be quickly created and readily sustained.

Itis also concerned with a construction for maintaining the temperature on the outside of the chimney construction below the safe limit established for the combustible construction with which it may be in contact, even when the heating device is discharging its products of combustion into the chimney at excessive temperatures caused by overfiring and the like.

Important objects of the invention thus reside in the provision of a new apparatus for dynamically controlling the flow of heat in a chimney construction, and in a chimney construction arranged to embody the same, which method and construction minimize heat loss through the walls of the chimney, quickly attain and maintain with low entering flue gas temperatures a relatively high flue temperature, and automatically discharge excess heat from the chimney in the event of overheating of the furnace or the like.

Class B vents and Class A chimneys having approval of the Underwriters Laboratories as heretofore constructed have generally been made up of one-fourth inch thick cement asbestos pipe, laminated pipe including layers of metal and of insulation, tile pipe, and the like. Such chimneys are approved only for installations providing one inch or more of air space clearance around the chimney with respect to any combustible material. However there is no way to assure or adequately check that this clearance requirement of the certificate of approval has been adhered to by the installing contractor and that the expected degree of safety has been obtained. Hence many fires have resulted from such installations, as a result, for example, of overheated furnaces or the like.

These conventional chimneys moreover have a rather high longitudinal temperature gradient and do not heat quickly throughout their height. They thus frequently produce a poor draft when the fire is low and require throwing of much heat up the chimney to create a proper draft with consequent poor emciency.

Such conventional chimneys moreover adopt expensive constructions where they attempt to increase the traverse temperature gradient to comply, for example, with the Underwriters Laboratories requirements that chimneys in contact with combustible material have outer wall temperatures, regardless of the stack temperature in the flue, of no more than ninety degrees Fahrenheit above room temperature; i. e., no more than one hundred sixty degrees Fahrenheit'.

Such chimneys, when provided with insulated layers in an efiort to meet such requirements, have high inside surface temperatures, particularly in the event of overheating of the furnace. Such high temperatures are apt to be highly detrimental to the flue liners, especially near the point of entrance of the combustion gases.

With these shortcomings of the prior art in mind, the present invention has for further objects, severally and interdependently, the provision of a novel method and arrangement ina chimney construction for maintaining a high transverse temperature gradient through the chimney and a low temperature gradient longitudinally thereof; the provision of a pre-fabricated chimney orthe like having air spaces built into it in such a way need be made on the job for the protection of combustibles; the provision of a chimney construction which is heat-retaining at safe flue temperatures and heat-dissipating at flue temperatures which would be excessive in the case of ordinary constructions;v and the provision of an arrangement providing a chimney construction which quickly attains an eflicient operating temperature throughout its height with small heat loss up the chimney, and which efficiently and safely dissipates excessive heat and avoids overheating in the event of overheating of the furnace.

An additional object of the invention is to provide an arrangement in chimney construction facilitating attic ventilation in a novel manner; and a still further object is to provide a chimney construction capable-in itself of obviating condensation difliculties.

In the latter regard, it is known that slow heating of chimneys, and slow cooling thereof, result in condensation near the top of the chimney over extended parts of the heating and cooling cycle. The condensate frequency gravitates to hotter areas and re-evaporates, cyclically. This causes excessive acid attack on the chimney construction, particularly in intermittently-fired installations. The present invention provides a chimney construction of low heat capacity as compared to brick, masonry, and statically insulated constructions. cycle is thus accelerated with consequent decrease in acid attack on the chimney structures. Indeed, tests with chimneys constructed in accordance with Fig. l, with entering flue gas temperatures ranging from 400 F. to 1400 F., show u that throughout the entire temperature range the construction maintains a flue wall temperature at the top of the chimney higher than that the flue gas inlet, with intermediate just above that no special provisions The heating and cooling 3 diate portions, as is highly desirable in eliminating condensation.

Other objects and advantages of the invention will be apparent from the following detailed description of preferred embodiments of the invention, illustrative of the principles thereof.

With such objects the invention resides in the novel construction by which a thermo-siphon circulation of air is established in "channels surrounding the flue so as to increase the temperature gradient through the walls of the chimney construction as its flue temperature increases, and to reduce the temperature gradient lengthwise of the construction, and in the cooperating steps and structural features hereinafter described and claimed.

In the accompanying drawings exemplifying preferred embodiments of the invention:

Fig. 1 is a diagrammatic vertical section through a residential installation embodying one form of the invention.

Fig. 2 is a similar diagrammatic section of a second form thereof.

Fig. 3 is a diagrammatic horizontal section taken on the line 3-3 of Fig. 1.

Fig. 4 is a similar section of a modification.

In the exemplification of Fig. 1 there is shown, schematically, a portion of a small residence comprising a floor E, a roof 6, and a ceiling I. Betweenthe floor 5 and the ceiling i is a space R. of a height sufficient for one or more stories of ordinary rooms, while above the ceiling is an attic, spaceA. Below the floor 5 is a basement B.

The chimney construction embodying the method and arrangement of the present invention includes a flue 8. Surrounding the flue B is a first or inner casing 9 having a length slightly less than the height of the flue. The casing 9 is outwardly spaced from the flue 8 to provide an air space In between the flue and the casing.

A conventional heater H, which may be of the oil-., gasor-coal-fired type, for example, is shown as resting on the floor F of the basement. The lower end of the flue 8 in the form of Fig. 1 has a horizontally-disposed laterally-extending intake I I which is connected by means of an elbow I2 to receive the products of combustion from the heater H. In the form shownthe lower end of the flue 8 is positioned slightly below the intake II and may be provided withany auxiliary device, as exemplified by the drain tube 8:1, for example.

The roof 6 is provided with an opening I3 through which the chimney extends. In thepresent invention a casing or attic ventilating duct I4 is secured within the'opening I3 of the roof, extends upwardly, and is spaced outwardly from the casing 9, thus forming a covered chamber communicating with the atmosphere, and which may communicate both with the attic space and the outside atmosphere when attic ventilation is desired. In the form shown the cap or cover I5 is supported above the upper end of the casing I4 in spaced relation thereto so that there may be air communication between the interior of the casing I4 and the outside atmosphere. The cap I5 is apertured to receive the upper end of the flue '8 to which it is connected in sealing relation, and-is supported on'the structure inany suitable way. For example, it may be supported on and be held centered by legs Ma extending upwardly from the top edge of the casing I4, or otherwise.

In the form shown inFig. 1 there is, in addition to .thefirst or inner casing 9, a second or outer casing I6 which surrounds the inner casing in then the outer casing spaced relation thereto. This outer casing I 6 may be cylindrical as shownin Fig. 2, rectangular as shown at I6 in Fig. 4, or of any other configuration. At its upper end the casing l6 terminates in proximity to the upper end of the inner casings, and preferably in spaced relation therebelow by an amount determined in accordance with the character of the installation. That is to say, if the attic or upper space A is unoccupied I6 terminates somewhere above the floor I; and if the space A is to be occupied, then the casing preferably terminates above the ceiling of space A, as shown in Fig. 1.

In the form shown the ducts I8 and I8 communicate at their lower ends by way of an enclosed space I711. The space Ila in this illustrative embodiment is enclosed by the lower :end of the outer casing I6 and a closure, floor, or bottom I1 lying between the lower-endof the outer casing I8 and the flue 8. The communication may be established from the lower end :tof the space l8 between the casings to the :lower end of the space I I) within the inner casing in any suitable manner. As shown in Fig. l, 'for example, the inner casing may be supported so that its lower end terminates a slight distance,

above the casing bottom or closure -I:I. Such support may be effected by cutting away the lower edge of the casing 9 to leavesupporting feet 9a as shown in Fig. 1, orby any other known or special mode of supporting casing sections. Suitable braces (not shown) may be used to hold the casings in centered relation, when :needed. In the form shown in Fig. :1 the upper ends "of the casings '9 and I8 terminatebelow the upper end of the casing I4 so that air may escape from the air space may enter the space l8 between the. casings ;9 and I6.

As will be appreciated from thefollowing detailed description of the operation of this embodiment, this structural arrangement makes available a new method of controlling transverse flow of heat through the chimney by surrounding the flue with an intussusoepted tubular thermo-siphon air blanket communicatingv with the atmosphere at its top and the .thermo-siphon circulation of which is a function of the differential heating of the inner and outer tubular layers of the blanket by the flue.

Thus, as the heater His operated, combustion gases enter the fiue 8 at its bottom and, pass upwardly therethrough. The temperature .of the gases entering the flue heats the air in the space In within the first surrounding casing 9. This air is therefore caused to fiowupwardly transferring heat from the lower 8 to its upper portion. As the air in 'thespa-ce I0 rises, air in the space I8 is drawn downwardly and through the communication in the bottom space Ila, so .a countercurrent flowis setup in the spaces I0 and I 8. With the arrangement shown in Fig. 1 a part of the air thus circulated passes out under the cap or cover member I5, and part of it may re-circulate downwardly through the duct I8. Air from the attic or other elevated space A may also be drawn downwardly through the duct I8 and pass outwardly under the cap I5.

The flow of air in thermo-siphon circulation through spaces I8 and I3 on initiation of combustion is fairly rapid until the temperature of the air in the space I8 approaches that of the air in the space l0. When this condition arises the thermo-siphon air flow becomes small and I II to the atmosphere and air portion .of the .flue

the slowly-moving air layer around the flue 8 within air space Ill maintains the temperature of the flue at such point, from its bottom to its top, that the gas temperature therein is substantially uniform from the bottom to the top of the flue. The re-circulation of air, via the chamber 14, hastens and augments the reduction of thermosiphon imbalance.

As the temperature of the gases within the flue 8 increases due to overfiring, for example, the temperature of the air within the air space H) rises. This rise increases the draft through the space ID and accelerates the air flow upwardly therethrough and downwardly through the space I8. As the air which is drawn into the space 18 is cooler than that issuing from space l0, and as its rate of induction is increased by a rise in the flue temperature, the arrangement tends actually to reduce the temperature in the air space l8 in the event of overheating of the furnace.

As long as the temperature within flue 8 is within the normal operating limits of, say three hundred to four hundred degrees Fahrenheit for an oil-burning installation, or three hundred to eight hundred degrees Fahrenheit, depending on the condition of the fire, in the case of a coalburning installation, the flow of air through the spaces 18 and I8 stabilizes at a relatively slow rate, so that the temperature of the flue 8 is kept substantially even throughout its height; i. e., the longitudinal temperature gradient of the flue becomes very small. Such maintenance of temperature is s fflcient to maintain the necessary operating draft satisfactory heating purposes without excessive loss of heat up the stack.

As mentioned, when the temperature within the flue rises, the temperature of the air within the space I is increased thus augmenting the draft through the space H] and the eduction of air into the space l8. The increase of air flow downwardly through the space [8 cools the surface of the casing 9 and prevents any excessive rise in temperature of the casing [6. Because of this mode of operation the chimney may with safety be erected with the casing l6 positioned directly against inflammable material, as the wooden construction elements I9, 20, 2|, 22, 23, 24, 5 and 1, for example, such arrangements being exemplified in Figs. 1, 3 and 4. Because the heating of the descending air in the casing I8 opposes the thermo-siphon circulation, control of this heating, as by re-circulation, or control of the heat passing through the wall 9, may be adjusted to concur the minimum rate of air circulation with the normal stack temperature range desired in the installation.

Demonstration and actual testing have satisfactorily proven that with the method and construction exemplified in Fig. 1 heat loss through the walls of the chimney is slight and that substantially uniform temperature throughout the flue has been maintained.

The embodiment illustrated in Fig. 1 is considered particularly well adapted for maintenance of uniform stack temperatures, and satisfactory for preventing undue rise in temperature of the casing H5 in the event of overheating of the furnace. It is also well adapted for attic ventilation and draws a substantial part of the thermo-siphon circulated air from the attic space since the hotter air, particularly in the case of large temperature differentials, tends to stratify thereabove and escape under the cap 15.

at the heater for economical and Attic ventilation is particularly desirable dur ing summer weather for cooling of the attic space, and during winter for prevention of condensation therein. With the preferred arrangement of this invention, when the furnace is shut down, as during summer weather, attic ventila-- tion continues in an uninterrupted manner through the ventilating duct [4 and the communication with the atmosphere under the cap l5. With normal winter operation of the furnace (or with the so-called summer-winter hook-up for year round heating of water) a substantial quantity of air, say fifty to one hundred cubic feet per minute, may I8 and ID and a substantial proportion of this air may be pulled from the attic space and delivered to the outside atmosphere.

The heights of the casings forming the air ducts l9 and I8, and the character of flow of air between the casings l8 and 9 may be determined by calculation in accordance with any particular construction. The effective lengths of the inner and outer air ducts, defining the spaces 16 and i8, are preferably balanced so that under normal stack temperatures air flow is very small, for example, of the order of one cubic foot per second, or less. The testing above mentioned has established that the outside temperature of the outer casing I6 is considerably below the ignition points of wood normally used for low cost construction of houses and the like. One series of tests showed the temperature of the outer casing to be Fahrenheit with flue temperature of 800 Fahrenheit when simple sheet metai casings were employed. The construction of the present invention thus maintains the temperature of the outer casing well below the Fahrenheit limit set by the Underwriters.

requirements.

Freedom of communication between the low'-' er ends of the duct spaces l8 and ill and the resistance to air flow through the ducts has a direct influence on the circulation of air in the construction. Thus by adjusting this resistance, as by raising or lowering the lower end of the tube 9 or varying the height of the legs 9a thereunder, or by providing the communicating passage with a valve such as the sleeve valve shown in Fig. 2, for example, a satisfactory adjustment of flow resistance and air flow may be obtained.

As relatively cool air is continuously circulated against the lower end of the flue 8 which receives the hot products of combustion, excessive heating of the flue liner is avoided with consequent increase in its life. The flue or liner 8 may consist of stovepipe sheet, steel, coated with vitreous enamel, asbestos cement tubing, stovepipe tile or any other material suitable to withstand the contemplated flue temperature. The.

inner duct 9 may be formed of any suitable material and is preferably formed of aluminum tubing to take advantage of its reflective value in reducing transfer of heat through the chimney structure. This provision reduces heat trans fer to the outer duct l8 and reduces the flowretarding effect thereof. When it is desired to effect more of such retardation, for example in, certain cases in which the outer duct terminates at attic floor level while the inner duct extends.

to roof height, so that there is a wide difference in length of the two heated columns, the dividing wall between then may be of black iron or other form relatively efficient in heat transfer.

The outer duct It may be formed of any suit able material such as galvanized iron or the like,

circulate through the ducts.

aesgmc or advantageously may be formed of bright material such :as aluminum still further to reduce byits reflective value the :outer vwall temperature of-the construction, and .retain in the outer duct 18 the heat transferred thereto through the wall :9.

The :relative proportions-of the flue, inner casing, and outer casing smay be varied within v.wide limits so long as the free circulation of .air through :the spaces 1'8 and required by the new -method is provided for. For example, the three flues may be of galvanized stovepipe six, eight and ten inches 'in diameter, respectively, mutuallyspaced in anysuitable manner, :as by spacers, indentation'of thecasing Walls, or otherwise.

As above indicated the invention may be embodied .in other arrangements than those described herein to exemplify its principles. In the embodiment shownin Figs. 2 and .4, for example, the :fiuet' is surroundedby an inner casing 9 and an outer casing l6, and the communication between the passages .18 and I9 is regulated by azsleeve valve 25' controllable by a .knob 26' which is movable in a slot 2? in the casing wall I21. In this form the flue 8' exits through the bottomof the casing. In addition the air passing upwardly from duct H3 is dis-charged to'atmosphereabovea partition 28' underlying the cap or cover and the outer casing it isprovided with an adjustableextension 29' for regulating the proportion of attic air and outside air drawn into the space I8. With this arrangement and a relatively'tig'httattic space any overheating .of the furnace causes the induction of a considerable volume .of cool outdoor air into the duct 1 B'and further guards against excessive rise of temperature of the outer casing It. When the valve is provided theslot ,2? is preferably so formed that complete closing of the valve25' is prevented so that some thermo-siphon circulation is insured at all times.

Preferredforms of the method and apparatus of the invention have been described herein more or less precisely as to details. It is to be understood, however, that the invention is-notto be considered as limited thereby, that changes may be made in the arrangement and proportions of parts,.and that equivalents may be substituted without departing from the spirit and scope of the invention.

I claim .as my invention:

11. A chimney construction for thermo-syphon balanced draft comprising a flue, a plurality of thermo-syphon air flow ducts surrounding said flue and'outwardly spaced from it and from each other, said ducts having'substantially equal effective lengths and having air flow communication at their lower ends to provide for inducing of countercurrent air flow therein by heat transmitted laterally from said flue for increasing the transverse temperature gradient and decreasing'the longitudinal temperature gradient of said chimney, said flue discharging to the atmosphere at its upper end and said thermo-syphon ducts each communicating with the atmosphere at its upper end ahead of and separately from the discharge of the flue thereto so that circulation in said-ducts is not affected by the draftin said flue.

,2. ,A chimney construction for thermo-syphon balanced draft comprising a flue, ,a plurality of thermo-syphon air flow ducts surrounding said fluerandoutwardly spaced :from it and from each other, said ducts having substantially equal effectiveiengths and having :air flow communication at their lower ends to provide "for inducing of countercurrent air flow therein by heat transmitted laterally from said flue for increasing the transverse temperature gradient and decreasing the longitudinal temperature gradient of said chimney, said flue discharging to the atmosphere at its upper end and said .themo-syphon ducts each communicating with the atmosphere-at its upper end ahead of and separately from the discharge of the flue thereto so that circulation in said ducts is not affected'by the draft in said flue, the relative heights of said ducts being adjusted to regulate the thermo-syphon induction of draft therein 'by the heat transmitted laterally from said flue.

3. A chimney construction for thermo-syphon balanced draft-comprising a flue, a-plurality of thermo-syphon air :flow ducts surrounding said flue and outwardly spacedfrom it and from each othensaid ducts having substantially equal eifective lengths and having air flow communication at their lower ends to provide for inducing of countercurrent air flow therein by heat transmitted laterally from said flue for increasing the transverse temperature gradient and decreasing the longitudinal temperature gradient of said chimney, said flue discharging to the atmosphere at its upper end and said thermo-syphon-ducts each communicating with the atmosphere at its upper ,end ahead of and separately from the discharge of the flue thereto so that circulation in said ducts is notaffected by the-draft in said flue, the relative heights of said ducts and the degree of flow communication therebetween being adjusted to regulate the thermo-syphon induction of draft therein by the heat transmittedclaterally from said flue.

4. A chimney construction for thermo-syphon balanced draft comprising a flue, a plurality of thermo-syphon air flow ducts surrounding said flue and-outwardly spaced from it and from-each other,-said ducts having substantiallyequal effec tive lengths and having air flow communication at their lower ends to provide for inducing of countercurrent air flow therein by heat trans-;

mitted laterally from said flue for increasingthe transverse temperature gradient and decreasing the longitudinal temperature gradient of said chimney, said flue discharging to the atmosphere at its u per end and said thermo-syphon ducts each communicating with the atmosphere atits upper end ahead of ,andcseparately from the discharge of the flue thereto so that circulation in said ducts is not aflected by the draft in said flue, the degree of air flow communication between said ducts being adjusted to regulate the thermo-' syphon induction of draft .therein by the heat transmitted laterally from said flue.

5. A chimney construction,for connection to a furnace comprising a chimney flue, means positioned aboutsaid flue for confining upflowingair in heat exchange relation to said flue, means for confining downflowing air in heatexchange relation to and extending substantially throughout,

the length of said upflow air confining means, means for admitting-air to the upper end of said downflow confining freely from theupper end of said upflow confining means, said means for confining upflowingair delivering the-air to the atmosphere ahead of and independently of the flueand means establishing communication between the lower ends of said downflow and upflow confining means whereby downflow and upflow :columns equal height areprovided through which thermomeans and for delivering air,

of substantially fiue draft does not accelerate flow of air through ential heat transfer between the flue and the two said ducts; whereby heating of the flue causing otherwise substantially balanced columns and tin of t air in Said air upfi w pace unaffected by draft in the flue. stitutes the sole means acting to promote thermo- 6. A chimney construction according to claim 5 5 syphon circulation of air through said downfiow further comprising an air chamber positioned and upfiow spaces, while transfer of heat to said about the upper end of said air flow confining heat conductive inner duct in turn causing heatmeans, the downflow air confining means coming of the air in said air downfiow space conmunicating with said air chamber and said air Stitutes means acting o retard uch thermosyphon circulation, the net result of such differchamber provided with an opening in its lower ential heating therefor being that on substantial syphon flow of air occurs dependent on the differportion for communicating with an attic space.

7. A chimney construction according to claim r159 111 mp ture of the flue gases supplied to further comprising an air chamber arranged ad- Said Central flue the lag 1h heatmg 0f Sald heat conductive inner duct produces a time delay bejacent the upper ends of said air-flow confining means and receiving warmed air from said upfiow l5 tween the circulation promoting and retarding confining means and delivering air to said downactions so that the thermo-syphon circulation is flow confining means, said air chamber further first Speeded p as the cehtral flue heats and communicating with the surrounding atmosphere. then Slows down as the mher 9 becomes A chimney construction according to claim 7 heated and stabilizes at a rate determined by the in which said air chamber communicates both differential heating; h pen e f a P with the outer atmosphere and further provided culat'loh 9 F rm -syphon efiect of heating with an opening in its lower portion for communi- 0f the m the upfiow Space cohtnhutmg to cation with an adjacent attic space. rapid heating f the upp r pa f th flu w n 9 A chimney construction comprising a hot flue gases are first supplied to the flue and tral flue, an inner metallic duct element nearly thus reducing condensation in the pp r p rt of the fiue and promoting draft therein.

as long as the flue and surrounding and spaced therefrom throughout its height to define there- A chlmney COHStTuQnPn accordm? to 51mm with an annular air upfiow passage, an outer duct 1h Whlch the Open portmn of the mhel duct element surrounding and spaced fr Said inner adlacent the bottom closure constituting a restriction to further control the rate of thermometallic duct element throughout at least a major portion of its length to define therewith an an- Syphon cllculatlonnular air do nfl passage, said dow fl and 13. A chimney construction according to claim upfiow passages communicating at their bottoms 11, further comprising hood means surrounding and being otherwise closed except at their upper the upper end of the central hue and extending ends, a chimney box Surrounding the upper ends outwardly over, but in spaced relation to, the of said duct elements and flue and spaced thereupper open ends of both the and outer from, said chimney box having a top and comductsmunicating with the atmosphere below its top and A chlmney constructlon accordmg claim independently of the flue, whereby heating of t 13, further comprising a chimney box surrounding flue produces thermo-syphon circulation of air 40 Said ducts and having an upper edge portion through Said do fl and u fi w passage com minating in spaced relation to said hood means, trolled by the transmission of heat from said from, whlchhox thehuter duct h its suphly flue to said upfiow passage and by conduction of air and into which box the inner duct disthrough said metallic duct to said downfiow pascharges warmed 15. A chimney construction according to claim 14, in which said chimney box is further provided sage.

10. A chimney construction according to claim 9, in which said chimney box is further provided h ah Qpemhg 1h lower porhoh for commum' with an opening in its lower portion for communicahhg h ah athc cation with an attic Space 16. A chimney construction according to claim 11, further comprising a covered chimney box 11. A chimney construction comprising a central heat conductive flue for flue gases, an air surrouhdlhg the upper ,ehds of the lhhhr h circulating system in which air-flow is accelerated huter 2hr h fromwhlch the w w h h solely by difierential thermo-syphon action ar- 15 drawn h e h e upfiowmg air is disranged in surrounding relation to said flue; said charged Sam Chlmhey box Y ah hpehmg air circulating system comprising a, heat conduchear top thereof commumcahhg with the tive inner duct surrounding said flue throughout shrrouhdlhg af'smosphere "T' provlhhg for the greater part of its length in spaced relation hal reclrchahhg of the ch'mhated 1 thereto, said inner duct and flue defining an air 7' A h cohstmctmn .accordmg to 01.2mm upfiow space therebetween, said air circulating 1h Whmh h hh box 1s.further provide? System further comprising an outer duct sub with an opening m ts lower portion for communicating with an attic space.

rounding said inner duct and spaced therefrom,

said ducts defining an air downflow space there- ROBERT THULMAN- between, said air circulating means also comprising bottom closure means surrounding said flue References (Med m the me of thls patent and extending to said outer duct and closing the 55 UNITED ATES PATENTS space therebetween, and said inner duct having Number Name Date an open portion at its end adjacent said closure 2, 3 3 5 Fluor Sept-h 20, 1938 means establishing communication between said 5 3 7 Howle Sept-h 19, 1944 outer and inner air-flow spaces, the upper end of 2,381,548 McLaughlin 7, 1945 said air downflow space being in communication 2,446,729 Van Alstyne Aug. 10, 19 8 with the atmosphere, and the upper end of the air upfiow space discharging to the atmosphere FOREIGN PATENTS ahead of said flue at a location out of direct com- Number Country Date munication with said flue and at which the pres- 157 Great Britain Jan. 5, 1886 184,224 Switzerland Aug. 1, 1936 sure is not reduced by the flue draft, so that the 

